The present invention relates generally to catalyst compositions and processes for making and using the same; and more particularly, it relates to the formation of polymer supported or unsupported di- and triphenylated cation radicals useful for cation radical catalyzed Diels-Alder reactions.
Most of the catalyst reagents available to ionize organic molecules are extremely powerful oxidants which possess alternate reactivity modes with the organic molecule substrates. An exception to the conventional ionizing reagents which induce severe oxidation is the tris-(p-bromo-phenyl) aminium hexachloroantimonate reagent (TBPA), which is a clean one electron oxidizing agent, having the following structure: ##STR1##
Despite TBPA's selective oxidation capabilities, this reagent has not met with commercial success for industrial use as a cation ion radical catalyst. Factors which have suppressed widespread implementation of this catalyst reagent include the costly and inefficient synthesis of the catalyst reagent, its short shelf-life, the undesirable residue of difficult to remove amines upon use of the catalyst to generate cation radicals, and the difficulty presented in recovering the spent catalyst from the reaction product mixture.
The present invention envisions the attachment of an aminium function similar to TBPA to an insoluble polymer framework. The product catalyst of this invention provides a recyclable and hence inexpensive catalyst which coincidently avoids the problem of the undesirable residue. The catalysts of the present invention are termed di- or triphenylated cation radical polymers. They are air stable, dark colored polymers which can be stored indefinitely, are conveniently recovered from the reaction product by simple filtration, and can be readily recharged for recycle. Furthermore, these catalysts, both the polymer supported and unsupported, have been shown to generate with increased yield and efficiency a variety of organic cation radicals, many of which were previously unknown, such as those organic molecules synthesized via the cation radical catalyzed Diels-Alder reaction mechanisms.
The Diels-Alder reaction is one of the most useful synthetic reactions in organic chemistry. The diversity of molecular complexity available through such reactions exceeds that of most other organic reactions. This arises in part from the creation of two new carbon-carbon bonds formed in the end product. In the Diels-Alder reaction, two components (one called a diene and the other termed a dienophile) are combined to form a cyclic compound. The simplest example is the reaction between butadiene with ethene to yield cyclohexene. This reaction requires rather high temperatures and characteristically provides poor yields.
The conventional (uncatalyzed) Diels-Alder reaction, though versatile, has several noteworthy limitations [see for example, Sauer, J. Angew. Chem. Internat. Edit. 6:16-33 (1967)]. Perhaps most important is the extremely low reactivity of approximately neutral and electron rich dienophiles. Although Lewis acid catalysts can accelerate certain Diels-Alder reactions, apparantly by increasing the electrophilicity of the dienophile, the strategy seems to succeed only with electron deficient, oxygen containing, dienophiles. One of the guiding premises of the work underlying the present invention was that the electron rich class of dienophiles is just the one which is most subject to ionization and that the resulting cation radicals, being electron deficient, might be activated "dienophiles". The scope of the cation radical catalyzed Diels-Alder reaction would therefore complement that of the conventional Diels-Alder reaction.
The cation radical catalyzed Diels-Alder reaction involves the generation of a cation radical organic molecule using polymer supported or unsupported di- or triphenylated cation radical salts and subsequent reaction of the new cation radical with a neutral organic molecule. The sole recorded precedent for such a reaction seems to be the report [Penner et al, J. Amer. Chem. Soc. 92:2861 (1970)] that radiolysis of 1,3-cyclohexadiene yields both the Diels-Alder and photo dimers, presumably via pathways involving diene cation radicals [Barton et al, J. Chem. Soc. Perkin Trans. I:2055 (1975)]. A somewhat looser, but still encouraging precedent is the apparent cation radical catalyzed addition of oxygen to dienes. The [2+2] dimerization of N-vinylcarbozole to give a cyclobutane product is the sole additional published example [Ledwith, A. Accts. Chem. Res. 5:133 (1972)] of a likely cation radical pericyclic process of which the applicants are aware.
The cation radical Diels-Alder reactions implementing the di- or triphenyl Group VA element cation radical and substituted polymers thereof of the instant invention possess significant utility in the synthesis of biologically relevant substances including compounds of the terpenoid, steroid, and alkaloid series.